UpdateTargetSystemRequestModel : The body of PUT rotation/target-systems/{id} (spec
UpdateAutomaticTargetSystem / UpdateManualTargetSystem), which replaces the separate rename
and policy operations. Carries the target’s whole updatable shape: every field is written as
sent, so a caller editing one of them must send the others as they stand rather than omitting
them. The target’s method and kind are fixed at registration and cannot be updated here.